The subject matter disclosed herein generally relates to turbomachines. More specifically, the subject disclosure relates to fuel and air passages through fuel nozzles for turbomachines.
As requirements for gas turbine emissions have become more stringent, one approach to meeting such requirements is to move from diffusion flame combustors to combustors utilizing lean fuel and air mixtures using a fully premixed operations mode to reduce emissions of, for example, NOx and CO. These combustors are known in the art as Dry Low NOx (DLN), Dry Low Emissions (DLE) or Lean Pre Mixed (LPM) combustion systems. These combustors typically include a plurality of primary nozzles which are ignited for low load and mid load operations of the combustor. During fully premixed operations, the primary nozzles supply fuel to feed the secondary flame. The primary nozzles typically surround a secondary nozzle that is utilized for mid load up to fully premixed mode operations of the combustor. Secondary nozzles serve several functions in the combustor including supplying fuel for the fully premixed mode, supplying fuel and air for a pilot flame supporting primary nozzle operation, and providing transfer fuel for utilization during changes between operation modes. In pilot mode, fuel for the operation of the pilot is directed through a pilot fuel passage typically located in the center of the fuel nozzle and air to mix with the pilot fuel is provided via a plurality of pilot air passages surrounding the pilot fuel passage. During transfer operation of the fuel nozzle, additional fuel is urged through the nozzle and into the combustion zone through a group of transfer passages located in the nozzle separate from the pilot fuel passage as a distinct flow of fuel. When the nozzle is not in transfer mode, the current practice is to purge the transfer passages of fuel by flowing transfer air through the transfer passages. In this operation the pilot is surrounded by this flow of lower temperature purge air. Separate passages in the secondary nozzle for pilot fuel, transfer fuel and air, and pilot air result in a complex nozzle assembly. Further, the pilot of the typical nozzle is fuel limited due to the configuration of the pilot fuel and air passages, so that high reactivity fuels cannot be utilized in the pilot.